Mechanical Biological Treatment (MBT) & Zero Waste

by Joan Marc Simon

Many cities have built Mechanical Biological Treatment Facilities (MBT) during the last decade with the aim of reducing the waste to be finally dumped or burnt. The results depend on every case, but it is clear that MBT alone is not the solution. However, it can play a role in transitional strategies to reduce residual waste without having to depend on more expensive undesirable options such as incineration. A well-designed ideal Zero Waste strategy shouldn’t need MBT.

What are MBTs?

MBT covers a wide range of activities and technologies to deal with residual waste –i.e. waste that hasn’t been separated for recycling or composting-. As the name explains it is composed of a mechanical part -in which waste is mechanically separated to recover recyclables- and a biological part –to either compost or digest the organic fraction-.

There are three main outputs from an MBT plant are; recyclables –such as PET plastic that can be sent for recycling-, low quality soil –the biologically stabilised part is used for land reclamation, almost never for agriculture- and RDF, Refuse Derived Fuel, which is a mix of materials with a homogeneus calorific value to burn in incinerators or in some cement kilns.

MBT became popular with the entry into force of the Landfill Directive which obliged member states to reduce the biodegradable waste going to landfill. MBT has the capacity to reduce the volume and methane emissions from waste, plus since it is modular it allows some flexibility and is cheaper and faster to build than any other big scale centralised options.

The draw-backs of MBT are that the bad quality of the compost they produce; almost always too polluted to be applied as soil improver. As a consequence, some authorities see MBT as a way to meet recycling rates without actually recycling and the production of RDF aimed at being burnt in incinerators and cement kilns.

MBT in Europe

MBT have been used with different success in Europe. For instance, in Germany they have been in use for more than 10 years and albeit having obtained some good results, the bigger the plant the more malodours and bacteria for the neighbourhood. The experience has been proven that MBT is not necessary when biowaste collection works well and there is high quality separate collection of other waste-streams combined with a good product policy promoting prevention of chlorine/PVC, heavy metals and flame retardants.

In Barcelona, Spain, MBT facilities were called ecoparcs and have been in operation since 10 years. Although they have managed to considerably reduce the amount of waste sent to landfills and are recovering materials for recycling it is a fact that no good compost has come out of these facilities whilst their production of RDF has increased. In fact, after all the expensive investments in MBT the city of Barcelona ended up implementing separate collection of organics (2010) after realising that the only way to get good compost is with input coming from biowaste source separation.

MBT and Zero Waste

A Zero Waste strategy means that waste should be prevented and source separation should be maximised. If all the products in the market would be recyclable and properly separately collected there will be no waste and hence no need for MBT. If MBT has a place in a Zero Waste strategy is only when dealing with the current 20-30% of total municipal solid waste that can’t be source separated and collected. In these cases MBT can be a temporary solution but always keeping in mind the goal of continuing minimising the residual waste.

In fact, the real name for MBT in a Zero Waste strategy would be a combination of a Material Recovery Center together with a Zero Waste Research Center. In these facilties the recoverable materials are recovered and the few residuals left are stabilised so that they can be safely landfilled after they go through the Zero Waste Research Center which analyses the defects in design in order to work upstream to make them recyclable in the future.

One of the pillars of Zero Waste is source separation of organics –the only way to obtain clean high-quality compost- and experience proves that this can’t be replaced by MBT.

Hey Zerowasteeurope,
Thanks you for your post, Biological treatment of wastewater through biological filters has been in practice for a long time. It offers many advantages. Here in this article I mention the two basic groups of biological filters and present a few details about the design of relatively new form of one of these filters.
I look forward to your next post